1. Field of the Invention
The present invention relates to a technology for detecting a toner image transferred onto an endless belt member and detecting toner adhesion amount per unit area in the toner image.
2. Description of the Related Art
In general, an image forming apparatus such as a copier and a printer disclosed in Japanese Patent Application Laid-open No. 2002-006568 and Patent Application Laid-open No. 2001-034030 has been known, in which an electrostatic latent image is formed on an image carrier such as a photosensitive element by optically scanning the image carrier, and a toner image is formed by developing the latent image with toner. In the above type of image forming apparatus, if a setting of image forming conditions such as light intensity for forming the latent image on the surface of the image carrier and developing potential for developing the latent image with toner is not suitable for an environmental condition, desired image density of the toner image (toner adhesion amount per unit area) cannot be obtained.
In the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2002-006568, toner images formed on surfaces of photosensitive elements are transferred onto a recording medium that is conveyed while being held on a surface of a transfer belt (a belt member), and the following operation is performed at a predetermined timing. After forming toner patch images for detecting the image density on the photosensitive elements, the toner patch images are transferred onto the surface of the transfer belt while the recording medium is not held. The image density of the toner patch images formed on the transfer belt is detected by an optical sensor such as a reflection-type photodetector, and the image forming conditions are adjusted based on a result of detecting the image density. With this configuration, even if the environmental condition is changed, the image density can be kept stable by adjusting the image forming conditions.
The image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2002-006568 is a tandem-type full-color image forming apparatus. In the tandem-type image forming apparatus, a toner image is formed on each of a plurality of image carriers for different colors arranged along a surface of a belt member, and the toner images are transferred in a superposing manner onto the surface of the belt member or a recording medium held on the surface of the belt member. A full-color toner image can be formed by forming the toner images with different colors on the image carriers and superposing them on the surface of the belt member or the recording medium.
Another type of tandem-type full-color toner image forming apparatus is disclosed in Japanese Patent Application Laid-open No. 2001-034030. In this image forming apparatus, toner images with different colors are respectively formed on a plurality of photosensitive elements arranged along a surface of an intermediate transfer belt (a belt member), and the toner images are transferred in a superposing manner onto the surface of the intermediate transfer belt, thereby obtaining a full-color toner image. The image forming apparatus regularly detects a relative displacement of each toner image at a predetermined timing, and corrects a timing to start writing a latent image and the like based on a result of detecting the relative displacement. Specifically, a toner image for detecting the displacement is formed on each of the photosensitive elements, and the toner images formed on the photosensitive elements are sequentially transferred onto the intermediate transfer belt, thereby forming a pattern image for detecting the relative displacement on the intermediate transfer belt. The relative displacement of the toner images in the pattern image is obtained based on a difference in times at which the toner images are detected by the photosensors. Then, the timing to start writing a latent image on each photosensitive element is adjusted based on the detection result.
With this configuration, a color misalignment due to the relative displacement of each toner image can be reduced. In general, in the tandem-type image forming apparatus, if a lens, a mirror, or the like in an optical-scanning unit for forming a latent image thermally expands or contracts due to a temperature change, a relative displacement is caused at a position at which writing a latent image starts on each photosensitive element, which results in the color misalignment. The above-described image forming apparatus detects the relative displacement of each toner image based on the difference in times at which the toner images in the pattern image are detected by the photosensors, and adjusts the timing to start writing a latent image and the like based on the detection result, to avoid the color misalignment.
In the above technologies, the image forming apparatus detects the toner patch images on the belt member or the image density of the toner images based on the change in optical reflectance on the surface of the belt member. However, if a distance between the surface of the belt member and the optical sensors slightly varies due to a ripple in the movement of the belt member, accuracy in detecting the toner patch images and the image density is degraded. Therefore, it is desired that the photosensors detect the toner patch images and the image density at a belt supporting area in which the belt member is supported by a supporting roller because the belt member shows virtually no ripple in the belt supporting area. For example, in the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2002-006568, the belt supporting area is taken as a target detection area.
However, in many cases, an area for arranging the photosensors to detect the belt supporting area is used for other purposes, making it difficult to secure a space for the photosensors. If a supporting roller dedicated to suppressing a ripple of the belt member at a position opposite the photosensors is additionally provided, the perimeter of the belt member becomes long, so that the image forming apparatus becomes bulky.
The same problems may occur even when transmission-type photosensors are used instead of the reflection-type photosensors.